The Avalanche Review, VOL. 4, NO. 3, JANUARY 1986
Copyright © All Rights Reserved; AAA

WINTER WEATHER Weather Patterns for Major Avalanches
by Peter Schaerer

Definition: Major avalanches are much larger than the usual annually observed avalanches. They occur once in 5 to 20 years, often come as a surprise, advance well into valleys, cover highways and railways, and may damage structures.

Conditions for major avalanches: Large avalanches start with the failure of a snowslab and in order to grow to a large size require:

a. an internal low strength bond or layer of snow;
b. deep snow overlying the weak bond;
c. a moderate to high strength of snow allowing a failure to propagate over a wide area;
d. a trigger that adds stresses or decreases the strength of the weak bond;
e. unstable snow in the track adding to the mass when the initial slab is in motion.

A combination of weather factors determine these conditions. Snowfall seems to be most significant because it supplies the mass of the avalanches. Temperature and wind follow closely in significance for major avalanches, because they influence the presence of absence of weak bonds between snow layers, determine the strength of snow slabs, and usually are the trigger of natural avalanches.

A study of avalanches at Rogers Pass illustrates that the number of small avalanches per winter is well correlated with the total snowfall, but this is not the case for major avalanches. With respect to broad weather and flow types the frequent small avalanches generally are related to "coastal weather" and the erratic major avalanches to "continental weather". The two weather patterns are illustrated for Western Canada. Coastal weather: The weather is characterized by frequent storms, producing sustained snowfall and fluctuating but not extremem temperatures. The upper circulation is zonal with rapidly moving waves from west to east. The development of ridges and troughs in the Pacific is weak. The pattern may also be associated with an upper trough or a low in the Gulf of Alaska or the northwest coast of British Columbia. The motion of the waves often is influenced by the mountain ridges running northwest to southeast, making the path of the storms difficult to predict. For this reason the local snowfall could be heavy in one valley and light in an adjacent one.

The sustained snowfall and moderate temperatures do not allow weaknesses to develop in the snowpack such as surface hoar, loose faceted crystals, or depth hoar. Avalanches run frequently during the storms, but the snow gains strength rapidly allowing no deep instabilities to develop.

Continental weather: The weather is characterized by long, dry periods with clear sky followed by heavy precipitation. The dry periods are caused by a blocking ridge at the Pacific Coast. Arctic air usually advances south and west to the Coast, but even without the cold air the outgoing radiation of the snow cover produces low surface temperatures which in turn results in weak faceted grains at the surface hoar.

The stable weather pattern suddenly breaks down and gives way to a southwest flow of moist, warm air. Heavy snowfall results when the moist air is lifted across the mountains and when it comes in contact with the arctic air. Strong wind develops when the flow is associated with a deep low in the Gulf of Alaska. An observer at the surface would observe heavy snowfall with a gradual raise of temperature, sometimes a change to rain. This weather pattern has the necessary ingredients for major avalanches: weak snow layers formed during the dry period; heavy snowfall adding weight and mass; high temperatures, high humidity and wind producing stiff snow slabs; high temperature and drifting snow acting as triggers. The avalanche hazard can build up rapidly, all hell may break loose. The winter of 1978-1979 is an example of a continental pattern.

Conclusion: Most winters are a combination of coastal and continental patterns. Usually they show a general coastal pattern with a zonal flow but contain outbreaks into continental pattern. The winter of 19841985 in Western Canada is an example of a shift between the two patterns.

Major avalanches usually require a continental pattern with clear, dry weather followed by heavy precipitation and raising temperatures. Major avalanches caused by sustained snowfall from a zonal flow are possible but infrequent.

The Avalanche Review, VOL. 4, NO. 3, JANUARY 1986
Copyright © All Rights Reserved; AAA